1. Field of the Invention
The invention relates to a controller for a vehicle power transmission system that includes an electric differential unit which includes a differential mechanism, having an input shaft and an output shaft, and an electric motor and which allows controlling a differential state between the rotational speed of the input shaft coupled to an engine and the rotational speed of the output shaft by controlling an operating state of the electric motor coupled to a rotating element of the differential mechanism, and a transmission unit that is arranged in a power transmission path between the electric differential unit and a drive wheel, and the vehicle power transmission system transmits power of the engine to the drive wheel. More specifically, the invention relates to a technique for suppressing an increase in engine rotational speed when the transmission unit shifts speeds.
2. Description of the Related Art
There is a known vehicle power transmission system that includes an electric differential unit. The electric differential unit includes a differential mechanism, having an input shaft and an output shaft, and an electric motor. The electric differential unit allows controlling a differential state between the rotational speed of the input shaft coupled to an engine and the rotational speed of the output shaft by controlling an operating state of the electric motor coupled to a rotating element of the differential mechanism. In the above vehicle power transmission system, the differential mechanism is, for example, formed of a planetary gear set. Then, a major portion of the power from the engine is mechanically transmitted to a drive wheel by the differential action of the differential mechanism, and the remaining portion of the power from the engine is electrically transmitted from a first electric motor to a second electric motor through an electrical path. Thus, the differential state is electrically controlled, that is, the speed ratio of the electric differential unit is appropriately varied. In this manner, the vehicle power transmission system is controlled to drive the vehicle while maintaining the engine at an optimally rotational state. This can improve fuel economy. Japanese Patent Application Publication No. 2006-335127 (JP-A-2006-335127) describes an example of the above vehicle power transmission system. JP-A-2006-335127 describes a technique for suppressing a shift shock by means of a torque reduction and an increase in engine rotational speed using an electric motor (or an engine) when the transmission unit shifts speeds.
Incidentally, in the vehicle power transmission system described in JP-A-2006-335127, torque reduction control is carried out using an electric motor (or an engine) when the transmission unit shifts speeds. However, for example, when the level of charge of an electrical storage device reaches an excessive state of charge in a state where it is impossible to reduce torque by the engine because of a decrease in engine coolant temperature, or the like, a torque reduction amount by the electric motor is limited. This may cause a situation where a shift shock is not sufficiently removed. For this reason, the engine rotational speed is increased to use the inertia of the engine, thus ensuring a torque reduction amount. However, when a torque reduction amount by the electric motor is limited as described above, an amount of increase in engine rotational speed increases. Therefore, there is a possibility that the engine rotational speed may reach a high rotational speed region (overrevolution). In addition, as the engine rotational speed reaches the high rotational speed region, an electronic throttle valve is closed to limit an engine torque in order to reduce the engine rotational speed, and the engine torque limitation may possibly cause a shift shock. However, there has been taken no effective measures against the above problem.